Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11804273) and the Key Science and Technology Program of Shaanxi Province, China (Grant No. 2019GY-170).
Received Date:01 July 2021
Accepted Date:26 July 2021
Available Online:17 August 2021
Published Online:05 December 2021
Abstract:The interaction between noble metal nanowires can induce the local surface plasmonic resonance effect, thereby enhancing the distribution of electric field in the nanostructures, which is of very important significance in improving the fluorescence characteristics and enhancing the sensitivity of sensors. In this study, we design several types of tetramers based on precious metals Ag nanostructures, including cylindrical and prismatic Ag tetramers, and by changing the arrangement and the rotation angle of prism nanowires, we simulate the rotation-angle dependent electric field distribution and electric field intensity of X component , and also discuss the physical mechanism of the relationship between the resonant peak position of absorption spectrum and the change of mode volume. The results show that in the Ag nanowires tetramer structure, the electric field in the cylindrical structure is not enhanced obviously, but the electric field in the prismatic structure is greatly enhanced, and an electric dipole resonance mode is produced in the gap between tetramers. The polarization of plasma resonant cavity revels that the morphology plays a decisive role in generating the hot spots, After changing both the combination mode of tetramer nanowires and the rotation angle of the four-prism, the local surface exciton resonance of the unrotated asymmetric tetramer structure is most ideal and has resonance intensity higher than the that of symmetrical four-prism structure. Therefore, our results provide a structural model and theoretical parameters for the enhancement of electric field intensity by local surface plasmon resonance effect. Keywords:Ag nanowire tetramer/ electric dipole resonance/ plasma resonance cavity/ local surface plasmon resonance effect
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3.结果与讨论图1为优化后的圆柱形与四棱柱Ag四聚体纳米线结构中的电场分布, 为了便于观察模态体积的变化, 对图中的电场值范围规范化到–10—10 V/m之间(非电场强度真实值), 根据场强值在XY面内接近或达到10 V/m的面积大小, 确定三维模型下共振模态体积的变化. 由图1(a)可以看出, 四个圆柱形Ag纳米线(C4)的组合使得间隙处的电场有一定程度的增强, 并由电场线分布情况可知, 多聚体结构使得匀强电场产生变化, 在纳米结构内部产生了电偶极子共振[17], 进一步证实了等离子共振耦合效应的出现, 但效果并不明显. 如图1(b)所示, 四个棱柱形Ag纳米线(Q4)的组合使得间隙处的电场急剧增强, 并且在形状较为尖锐的区域都出现了较强的电场分布, 且由电场线分布可以看出, 四棱柱四聚体结构可以在内部形成明显的电偶极子共振效应, 由静电近似[18]可知, 棱柱型纳米线四聚体等离激元共振的极化率远远高于圆柱形纳米线四聚体. 相较于单个纳米线产生的局域表面等离子体, 多聚体结构会在间隙内产生共振耦合效应, 其本征共振模式的强度正比于电场波动方程的振幅, 图2所示为四聚体结构间隙处的电场分布, 棱柱形Ag四聚体共振峰明显强于圆柱形结构, 说明棱柱形四聚体内部的极化强度要远远高于圆柱形. 通过改变四聚体单胞的形貌结构, 可得到急剧的场增强变化, 证实了形貌对于局域表面等离激元共振的形成起到决定性作用, 在纳米结构中的尖点、突起和岛状等结构中, 伴随着“热点”效应[19], 即量子尺寸效应, 这种限制效应会极大增强局域电场, 进而产生共振耦合. 量子尺寸效应可表示为[20] 图 2 圆柱形Ag纳米线四聚体与四棱柱四聚体结构中的电场X分量分布图 Figure2. Distributions of the X-component of the electric field in the cylindrical tetramer and quadrangular tetramer structures of Ag nanowires.
由于热点的产生不仅与尖点和突起等微纳形貌有关, 且与纳米结构的间隙也有密不可分的关系, 通过旋转棱柱形纳米线, 改变尖点的角度, 进而调制圆柱形纳米线与棱柱形尖点之间的耦合距离. 图4所示为C3Q1, C2Q2-Ⅰ, C2Q2-Ⅱ, C1Q3-up, C1Q3-down以及Q4在单个棱柱形纳米线逆时针旋转15°的四聚体结构电场分布图, 由于逆时针和顺时针旋转15°的电场分布几乎一致, 所以图4中仅展示逆时针旋转的情况, 而在考虑对称性的情况下, C1Q3四聚体组合结构中, 会存在两种情况, 即分别旋转对角线上的棱柱纳米线(C1Q3-up)及右下角的纳米线(C1Q3-down). 当棱柱形纳米线经过旋转与圆柱形纳米线之间的间隙变大, 共振耦合效应将会减弱, 尤其在C1Q3-up和C1Q3-down的对比更为明显, 共振腔仅在两者接触部位产生. 图 4 (a) C3Q1, (b) C2Q2-Ⅰ, (c) C2Q2-Ⅱ, (d) C1Q3-up, (e) C1Q3-down 与 (f) Q4单个棱柱形纳米线旋转15°的四聚体结构电场分布图 Figure4. Electric field distributions of (a) C3Q1, (b) C2Q2-Ⅰ, (c) C2Q2-tangent, (d) C1Q3-up, (e) C1Q3-down and (f) Q4 etramer structure after a single prismatic nanowire rotating 15°.
图5所示为仿真区域中心截线, 即未旋转与旋转棱柱形四聚体间隙共振位置(4 nm与–4 nm)处的电场强度分布图. 随着棱柱形纳米线的加入, 调制了共振耦合区域, 改变了等离子体腔的模态体积, 进而影响了共振强度. 纵向平行排列(C2Q2-Ⅰ)的棱柱四聚体相比对角排列(C2Q2-Ⅱ), 仅在极化方向上产生了局域表面等离子体, 共振耦合效应并不明显, 同时, 从C3Q1和C1Q3的对比中可知, 圆柱形与棱柱形产生的热点要明显强于双棱柱形貌结构, 且在C1Q3四聚体结构中, 共振强度最为明显, 强于Q4结构. 图5(b) 中展示的间隙处电场共振强度更加直观的说明单个棱柱形纳米线经过旋转之后对共振腔模式的破坏及调制作用, 不仅对共振位置产生了影响, 同时共振强度减小了大约1/4, 可见形貌对于极化率因子的影响[22], 当纳米结构的接触半径R <5 nm时, 介电常数的极化依赖便不可忽略, 随着极化率提升至无穷大, 等离子激元便发生共振耦合. 图 5 (a)未旋转纳米线四聚体与(b)单个棱柱形纳米线旋转15°的四聚体结构间隙处电场分布图 Figure5. Electric field distributions diagram at the gap between the tetramer (a) without rotation and (b) after 15° rotation of a single prismatic nanowire.